The present invention generally relates to display driving methods and apparatuses, and more particularly to a display driving method and apparatus suited to drive a plasma display panel (hereinafter simply referred to as a PDP).
The PDP is expected to become one of the display devices of the next generation and to replace the conventional cathode ray tube (CRT), because the PDP can easily realize reduction in the thickness of the panel, reduction in the weight of the panel, flat screen shape and large screen.
A PDP which makes a surface discharge has been proposed, and according to such a PDP, all pixels on the screen simultaneously emit light depending on display data. In the PDP which makes the surface discharge, a pair of electrodes are formed on an inner surface of a front glass substrate and a rare gas is filled within the panel. When a voltage is applied across the electrodes, a surface discharge occurs at the surface of a protection layer and a dielectric layer formed on the electrode surface, thereby generating ultraviolet rays. Fluorescent materials of the three primary colors red (R), green (G) and blue (B) are coated on an inner surface of a back glass substrate, and a color display is made by exciting the light emission from the fluorescent materials responsive to the ultraviolet rays. In other words, fluorescent materials of R, G and B are provided with respect to each pixel forming the screen.
FIG. 1 is a diagram showing an example of a gradation driving sequence of the PDP which makes the surface discharge as described above. As shown in FIG. 1, 1 field which is the time in which 1 image is displayed, is divided into a plurality of sub fields, and the gradation display of the image is made by controlling a light emission time (hereinafter referred to as a sustain time) in each sub field. 1 sub field is made up of an address display-time in which a wall charge is formed with respect to all of the pixels which are to make the light emission within the sub field, and the sustain time in which a luminance level is determined. In this specification, the “wall charge” refers to the charge induced at the dielectric layer and the protection layer on the electrodes and at the surface of the fluorescent materials. For this reason, if the number of sub fields within 1 field increases, the number of address display-times increases depending on the increase of the sub fields, thereby reducing the relative sustain times that may be provided for the light emission and deteriorating the luminance of the screen.
Accordingly, in order to increase the number of displayable gradation levels of the PDP using the limited number of sub fields, the PDP is generally driven with the sustain time proportional to the bit weighting as shown in FIG. 1. In the case shown in FIG. 1, 1 field is made up of 6 sub fields SF1 through SF6, and the display is made with 64 gradation levels based on 6-bit image data corresponding to each of the sub fields SF1 through SF6. For the sake of convenience, the sustain times within the sub fields SF1 through SF6 are indicated by the hatching to indicate the ON state, that is, the light emission state. The duration ratios or length ratios of the sub fields SF1 through SF6 are set to satisfy a relation SF1:SF2:SF3:SF4:SF5:SF6=1:2:4:8:16:32. In this particular case, 1 field is approximately 16.7 ms.
When displaying a moving image on the PDP using the above described gradation driving sequence, a contour of an unnatural color which originally does not exist is generated at the surface of the moving object in the image due to the residual image effect and the like of the human eyes. In this specification, such a contour of the unnatural color caused by the residual image effect and the like will be referred to a “pseudo contour”. The pseudo contour becomes particularly conspicuous when a person on the screen moves. The pseudo contour appears to the human eyes as a band of green or red color at the skin-colored portion such as the face of the person, and the pseudo contour greatly deteriorates the image quality.
A description will be given of the mechanism by which the pseudo contour is generated in conjunction with FIGS. 2 through 7, by referring to phenomenons (1) through (3). For the sake of convenience, FIGS. 2 through 7 show a case where 1 field is made up of 4 sub fields. In addition, in FIGS. 2 through 5, the length ratios the sustain times in the 4 sub fields are set to 1:2:4:8 in the sequence in which the light emission state is determined. In FIGS. 6 and 7, the length ratios of the sustain times in the 4 sub fields are set to 1:4:8:2 in the sequence in which the light emission state is determined. In FIGS. 2 through 7, those sustain times which assume the light emission state, that is, the light emission state, are indicated by the hatching. In this case, it is possible to display 16 gradation levels from a level 0 to a level 15. In FIGS. 2 through 7, the abscissa indicates the time, and the ordinate towards the upward direction indicates the left side of the screen and the ordinate towards the downward direction indicates the right side of the screen. In addition, the numerals indicated along the ordinate indicate the luminance level. The illustration of the address display-times with the sub fields, that is, the non-light emission times, is omitted in FIGS. 2 through 7.    Phenomenon (1):
In a first case, it is assumed for the sake of convenience that a Gray scale image which becomes brighter from the left towards the right of the image, that is, an image in which the luminance increases from the left to right of the image, is displayed on the PDP. If this image continuously moves towards the left of the screen by an amount corresponding to 1 pixel for every 1 field, a portion where the light becomes sparse appears to the human eyes. On the other hand, if this image continuously moves towards the right of the screen by an amount corresponding to 1 pixel for every 1 field, a portion where the light becomes dense appears to the human eyes. These sparse and dense portions where the light appears sparse and dense, respectively, occur when the human eyes focus on the moving object displayed on the screen, because the human eyes follow the moving direction and moving speed of the moving object and the visual point moves along loci indicated by bold arrows in FIGS. 2 and 3. FIG. 2 is a diagram showing a locus of a visual field of human eyes in a case where a Gray scale image in which the luminance increases from the left to right of the image is displayed on a PDP and this image continuously moves towards the left of the screen by an amount corresponding to 1 pixel for every 1 field.
FIG. 3 is a diagram showing a locus of the visual field of the human eyes in a case where a Gray scale image in which the luminance increases from the left to right of the image is displayed on a PDP and this image continuously moves towards the right of the screen by an amount corresponding to 1 pixel for every 1 field.    Phenomenon (2):
In a second case, it is assumed for the sake of convenience that a Gray scale image which gradually becomes brighter from the left towards the right of the image, that is, an image in which the luminance gradually increases from the left to right of the image, is displayed on the PDP. If this image moves towards the left of the screen at a constant speed by an amount corresponding to 1 pixel for every 1 field, a portion where the light becomes sparse appears to the human eyes. On the other hand, if this image moves towards the right of the screen at a constant speed by an amount corresponding to 1 pixel for every 1 field, a portion where the light becomes dense appears to the human eyes. These sparse and dense portions where the light appears sparse and dense, respectively, occur when the human eyes focus on the moving object displayed on the screen, because the human eyes follow the moving direction and moving speed of the moving object and the visual point moves along loci indicated by bold arrows in FIGS. 4 and 5. Such a phenomenon occurs when the image displayed on the screen during 1 field moves at a high speed or at a low speed.
FIG. 4 is a diagram showing a locus of the visual field of the human eyes in a case where a Gray scale image which has a gradation with a width of 3 pixels and in which the luminance gradually increases from the left to right of the image is displayed on a PDP and this image moves at a constant speed towards the left of the screen by an amount corresponding to 1 pixel for every 1 field. FIG. 5 is a diagram showing a locus of the visual field of the human eyes in a case where a Gray scale image which has the gradation with the width of 3 pixels and in which the luminance increases from the left to right of the image is displayed on a PDP and this image moves at a constant speed towards the left of the screen by an amount corresponding to 3 pixels for every 1 field.    Phenomenon (3):
In a third case, it is assumed for the sake of convenience that a Gray scale image which becomes brighter from the left towards the right of the image, that is, an image in which the luminance increases from the left to right of the image, is displayed on the PDP. In this case, even when the sub field structure is changed and the length ratios of the sustain times in the 4 sub fields are set to 1:4:8:2 in the sequence in which the light emission state is determined, as shown in FIGS. 6 and 7, portions where the light becomes sparse and dense to the human eyes occur if this image continuously moves towards the left of the screen by an amount corresponding to 1 pixel for every 1 field. On the other hand, portions where the light becomes dense and sparse to the human eyes occur if this image continuously moves towards the right of the screen by an amount corresponding 1 pixel for every 1 field. These portions where the light appears sparse and dense or vice versa, respectively, occur when the human eyes focus on the moving object displayed on the screen, because the human eyes follow the moving direction and moving speed of the moving object and the visual point moves along loci indicated by bold arrows in FIGS. 6 and 7.
FIG. 6 is a diagram showing a locus of the visual field of the human eyes in a case where a Gray scale image in which the luminance increases from the left to right of the image is displayed on a PDP by changing the sub field structure from that of FIGS. 3 through 6 and this image moves towards the left of the screen by an amount corresponding to 1 pixel for every 1 field. FIG. 7 is a diagram showing a locus of the visual field of the human eyes in a case where a Gray scale image in which the luminance increases from the left to right of the image is displayed on a PDP by changing the sub field structure from that of FIGS. 3 through 6 and this image moves towards the left of the screen by an amount corresponding to 1 pixel for every 1 field.
The above described phenomenons (1) through (3) become particularly notable at the luminance levels where the sub fields of the light emission state greatly deviate along the time base (or axis). Hence, in the case where the display can be made using 16 gradation levels as shown in FIGS. 2 through 7, the phenomenons (1) through (3) become notable at the portion where the luminance level changes from the level 7 to the level 8 and at the portion where the luminance level changes from the level 8 to the level 7.
Next, a description will be given of the mechanism by which the pseudo contour becomes visible to the human eyes when the moving object displayed on the screen is a person's face having the skin tone, for example, based on the phenomenons (1) through (3).
For the sake of convenience, it is assumed that the ratios of the luminance levels of R, G and B for the skin tone is R:G:B:=4:3:2. In this case, the gradation characteristic becomes as shown in FIG. 8. In FIG. 8, the ordinate indicates the signal level in arbitrary units, and the abscissa indicates the luminance level. In FIG. 8, the luminance of the skin tone becomes darker towards the left and brighter towards the right. Portions where the light appears sparse or dense to the human eyes exist depending on the moving direction of the moving object displayed on the screen, and in FIG. 8, a portion indicated by a black circular mark where the luminance level is R1=0.5 and a portion indicated by a black circular mark where the luminance level is G1=0.5 correspond to such portions.
FIG. 9 shows a case where the moving object displayed on the screen moves towards the left of the screen, where the moving object has the skin tone having the above described ratios of the luminance levels of R, G and B. An upper half of FIG. 9 indicates the display on the screen, and a lower half of FIG. 9 indicates the luminance levels of each of the primary colors R, G and B. In FIG. 9, an oval shaded portion corresponds to the moving object which has the skin tone, and it is assumed that the luminance becomes higher towards the central portion of the oval portion. The signal characteristics of R, G and B indicated in the lower half of FIG. 9 are with respect to the double lines passing the central portion of the oval portion.
In the case of the sub field structure described above, the portion where the luminance level is R1 in FIG. 8 corresponds to portions indicated by P1 and P4 in FIG. 9. Accordingly, when the moving object moves towards the left of the screen and the human eyes follow this moving object, the light becomes sparse at the portion P1 while the light becomes dense at the portion P4. In addition, the portion where the luminance level is G1 in FIG. 8 corresponds to portions indicated by P2 and P3 in FIG. 9. Thus, when the moving object moves towards the left of the screen and the human eyes follow this moving object, the light becomes sparse at the portion P2 while the light becomes dense at the portion P3. In other words, the luminance level of R decreases at the portion P1 and a band of G (or B) moves towards the left of the screen, and the luminance level of G decreases at the portion P2 and a band of R (or B) moves towards the left of the screen. On the other hand, the luminance level of G increases at the portion P3 and a band of G moves towards the left of the screen, and the luminance level of R increases at the portion P4 and a band of R moves towards the left of the screen.
As a result, even if the moving object has a skin tone with a smooth or gradual change in gradation level, a band of a color which originally does not exist appears to the human eyes at the contour portion of the moving object. As described above, this pseudo contour is notably generated at the skin tone portion such as the person's face and makes the image extremely unnatural, thereby deteriorating the image quality.
On the other hand, in the PDP using the sub field structure described above, a change in a least significant bit (LSB) of the image data may result in a large change of the position (time) on the time base of the sub field having the light emission state depending on the luminance level. This large change in the position of the sub field having the light emission state generates a flicker having a frequency lower than the frame frequency which is 60 Hz, for example, thereby deteriorating the image quality.
When it is assumed that the length ratios the sustain times in the 4 sub fields which make up 1 field are set to 1:2:4:8 in the sequence in which the ON state is determined, it is possible to display 16 gradation levels from the level 0 to the level 15, as described above. However, if the luminance level of a pixel changes between the levels 7 and 8 for every field, that is, changes to levels 7, 8, 7, 8, . . . for every field as shown in FIG. 10, a luminance level change of 0 (all black), 15 (all white), 0 (all black), 15 (all white), . . . appears at a frequency of 30 Hz to the human eyes, thereby generating the flicker.
Hence, the generation of the flicker is conspicuous at the portions where the sub fields having the light emission state greatly changes on the time base. When a pixel of an original image represented by 256 gradation levels has a luminance level in a vicinity of 128 and is displayed on a PDP which can display 16 gradation levels, the flicker is easily generated due to quantization error, video noise and the like even though the original image is a still image, and the image quality is deteriorated as a result.
Therefore, when the conventional gradation driving sequence is used for the PDP, a band of a color which originally does not exist appears to the human eyes at the contour portion of the moving object, even when the skin tone of the moving object undergoes a gradual change in gradation, thereby resulting in a problem in that the pseudo contour is visible to the human eyes. In addition, the pseudo contour is notably generated at the skin tone portion such as the person's face, and the image becomes extremely unnatural and the image quality is deteriorated thereby.
On the other hand, there is another problem in that the generation of the flicker is notable at portions where the sub fields having the light emission state greatly change on the time base. For example, when a pixel of an original image represented by 256 gradation levels has a luminance level in a vicinity of 128 and is displayed on a PDP which can display 16 gradation levels, the flicker is easily generated due to quantization error, video noise and the like even though the original image is a still image, and the image quality is deteriorated as a result.